The use of optical fibers for various telecommunication systems and other applications has become increasingly prevalent. It has led to the creation of a whole burgeoning industry over the past few years. In the foreseeable future the field of fiber optics is expected to experience a quantum leap in both stature and product diversification.
As is well known in the art, optical fibers are typically hair thin structures, capable of transmitting light signals at relatively high rates and with relatively low signal loss. They are believed to be ideally suited to the high requirements of digital transmission and, hence, are well matched to the evolving worldwide data transmission network.
In order to achieve efficient optical fiber data transmission networks, various types of so-called optical components and devices have been developed. Examples of optical components and devices commonly used in optic communication systems and instrumentation include couplers, wavelength division multiplexers/demultiplexers, filters, attenuators, filter switches, isolators and so forth.
Some optical components are particularly vulnerable to mechanical stresses. In certain instances, this vulnerability is directly linked to the inherent structure of the optical component. Indeed, some optical components being made of modified glass fibers, they are characterized by an inherent brittleness that can lead to mechanical failure. The adjacent sections of optical fibers coupled to the optical components are also often particularly vulnerable to mechanical stress, especially if they have been locally stripped of their protective coating.
Aside from potentially causing breakage of the optical component and/or the adjacent sections of optical fiber attached thereto, bending or other mechanical stresses imposed thereon may potentially cause degradation of optical performances by inducing changes in optical signal strength and content.
Accordingly, for a given optical component to function reliably in practical environments, it is conventionally enclosed in a generally rigid protective casing or housing. Each strip of optical fiber coupled to the optical component extends out of the housing through a corresponding fiber aperture formed in the housing. A sealing component or assembly is sometimes used to seal the spacing between the peripheral edge of the fiber aperture and the peripheral surface of the fiber extending therethrough.
The combination of the rigid housing and of the sealing component or assembly is often referred to in the art as an optical component package. These optical component packages protect the optical components and adjacent strips of optical fiber attached thereto during both installation and operation.
Some optical components such as certain types of couplers are also particularly sensitive to environmental influences including hostile temperatures, radiation and contact with so-called contaminants such as the hydroxyl ion [OH] associated with water. Moisture in particular may prove to be detrimental to some optical components for several reasons.
For example, because of their inherent thermal motion, the oxygen and hydrogen atoms in the hydroxyl ion [OH] vibrate at 2.73 μm and produce a relatively large absorption loss at this wavelength. Of course, 2.73 μm is outside of the band where fibers are typically used and the loss peak at this wavelength is of relatively minor significance. However, unfortunately, overtones of this wavelength produce three loss peaks within the 700 to 1600 nm band, and these can interfere with signal transmission.
Moisture also tends to exaggerate surface flaws and, hence, lower tensile strength of the optical component and strips of optical fiber coupled thereto. Furthermore, when exposed to gaseous hydrogen, some optical components show an increase in optical loss that can be either temporary or permanent, depending on whether the hydrogen simply diffuses into the component and then leaves, or remains long enough to react chemically with the fiber.
Accordingly, most optical component packages are provided with housings made of generally contaminant impervious material. One of the main sources of contaminant ingress within the protective housing of prior art optical component packages remains the interface between the fiber aperture peripheral edge and the outer surface of the section of fiber extending through the fiber aperture.
Some prior art packages include housing tubes made of stainless steel or other suitable material. The housing tubes are typically used both as protective covering and as a support for the optical component. Typically, the housing longitudinal ends define fiber apertures that are generally oversized relative to the fiber diameter. A relatively large spacing is hence formed between each fiber aperture peripheral edge and corresponding fiber outer surface.
Various methods for providing a seal against contaminants between the fiber aperture peripheral edge and the adjacent fiber outer surface are disclosed in the prior art. For example, when a relatively large spacing is defined between a fiber aperture peripheral edge and a corresponding fiber outer surface the spacing is sometimes filled with a relatively short sealing plug made of an epoxy-based material. The epoxy-based material is typically applied at both ends of the tube not only to act as a seal but also to secure the sections of optical fiber extending from the tube.
One of the main drawbacks associated with the hereinabove mentioned type of prior art optical component packages is that the relatively short and wide epoxy-based sealing plugs typically do not provide an efficient barrier against moisture ingress into the tube ends. Indeed, it is well known that the physical characteristics of cured epoxies are such that they have a tendency to absorb and allow ingress of moisture.
One of the design-modifiable factors regulating the diffusion of humidity through epoxy plugs is the surface over length (A/L) ratio of the opening to be sealed. In order to reduce water ingress, the housing should preferably be designed so as to minimize the A/L ratio. Most prior art packaging devices using silicone sealing plugs however are not designed so as to reduce the A/L ratio and, hence, offer relatively poor protection against humidity ingress.
Another main drawback associated with the hereinabove disclosed type of prior art optical component packages is that the optical component and at least a section of the optical fiber attached thereto are often susceptible of being subjected to mechanical stresses if the optical component package is subjected to temperature variations. Temperature variations may prove to be particularly deleterious in situations wherein the optical component and/or the strips of optical fiber attached thereto are solidarly anchored at spaced apart locations to a rigid sealing component and to a fixed mounting fixture within the housing.
In many applications employing optical components or devices, design specific mounting fixtures are utilized to precisely position, align or secure the strips of optical fiber to the optical components or devices. In this respect, maintaining the stability and relative position of optical fibers, components and devices, through the correct choice of materials, is particularly critical in that even minor relative movements between such elements may result in large variations or degradation in optical characteristics.
The thermally induced mechanical stresses to which some optical components are subjected to are typically imputable to the mismatch between the coefficients of thermal expansion of the various components involved. Indeed, as is well known, when two dissimilar materials are joined together and subjected to thermal cycling, the difference in their coefficients of thermal expansion can induce relatively large structural stresses. Furthermore, depending on the exact geometry and material distribution, the induced structural stress fields, although often transient, may be quite complex. Some thermally induced stress concentration points may potentially damage relatively fragile microstructures.
In an attempt to reduce these thermally induced mechanical stresses, some prior art optical component packages have been designed with a protective housing made out of a material having a coefficient of thermal expansion generally similar to that of the optical fiber. However, the sealing plugs and the fiber to which they are bound typically remain made out of generally dissimilar classes of material, namely a polymeric material such as epoxy and a glass-based material such as silica leading to thermally induced mechanical stresses. These thermally induced mechanical stresses are susceptible of exceeding the adhesion strength of the seal, hence potentially resulting in delamination during thermal cycling.
Other types of prior art optical component packages have resorted to using relatively resilient materials such as silicone for forming the sealing plug and/or the fiber-to-housing attachment component. The more resilient materials provide the advantage of being less susceptible of generating localized thermally induced mechanical stresses since they allow for a certain degree of deformation and hence act as decoupling components.
However, the use of silicone is associated with some important drawbacks such as a relatively poor sealing efficiency. Indeed, sealing plugs made out of silicone may not form strong chemical bonds with the outer surface of the optical fiber. Furthermore, silicone releases cyclosiloxane, a contaminant known to induce drift or wavelength shift in the optical components.
In order to circumvent some of the hereinabove mentioned problems associated with some common types of prior art optical component packages, sealing of the housing fiber apertures is sometimes carried out by a conventional soldering operation. To this purpose, it is necessary to first remove the acrylate coating from the section of the fiber extending through the fiber aperture. The uncoated fiber length is then carefully cleaned and surface-metallized
For example, in a conventional metalization step, an inner layer made of titanium or chrome is typically bound to the outer surface of the fiber. This first layer is then typically covered by an intermediate layer of nickel or platinum and by an outer coating of gold in order to ensure appropriate wettability of the fiber by the solder.
Although solder seals are generally considered as being relatively efficient in preventing contaminant ingress, they nevertheless suffer from numerous drawbacks. The surface metalization process is both tedious and time consuming. Also, the solder surface-metallized portion of the fiber typically exhibits a relatively low tensile strength. This reduction in the tensile strength of the surface metallized portion may result from factors inherent to the process including the creation of micro-cracks during the removal of the fiber coating and stresses generated during both metalization and soldering steps.
Another type of sealing structure disclosed in the prior art for sealing the fiber apertures of packaging housings is commonly referred to as a glass-seal structure. Glass seal structures rely on glass-based material as the sealing agent. Some of the glass-seal structures suffer from the same drawbacks as the hereinabove disclosed other types of seals. In particular, some of the prior art glass-seal structures suffer from drawbacks associated with mismatches between the coefficient of thermal expansion of the sealing plug and of the housing protecting the fiber.
However, the prior art has shown at least one example of a glass-seal structure advantageously using the discrepancy between the coefficient of thermal expansion of the sealing plug and of the housing for creating a relatively hermetic compressive sealing joint at operational temperatures. U.S. Pat. No. 5,664,040, naming Daniel P. Kramer and Thomas M. Beckman as inventors, issued Sep. 2, 1997 discloses an assembly including a strip of optical fiber extending from a housing referred to as a metal holder. A so-called sealing medium made of glass sealingly engages the metal holder to the fiber.
The components are selected to be of material having compatible thermal expansion coefficients (TEC). More specifically, the TEC of the metal holder is greater or equal to the TEC of the sealing glass. The TEC of the sealing glass is, in turn, greater or equal to the TEC of the optical fiber. Manufacturing of the assembly involves heating the sealing glass until the latter attains a molten state. The sealing glass is then allowed to cool while in an intermediate position between the metal holder and the optical fiber so as to form a relatively hermetic compressive and solid seal therebetween.
Although the glass-seal structure disclosed in U.S. Pat. No. 5,664,040 at least partially solves some of the hereinabove mentioned drawbacks associated with other prior art sealing structures, it nevertheless inherently suffers from other drawbacks prohibiting its use in certain settings.
Indeed, since the sealing material used is a glass-based material, the softening temperature of the latter is inherently relatively high. The document mentions the use of a sealing glass preferably having a softening temperature in the range of about 700° C. to 900° C. This inherently relatively high softening or temperature may prove to be unacceptable in certain settings, for example when heat sensitive optical components are packaged especially if thermally sensitive adhesives are also used in the package.
Furthermore, glass-based sealing materials have also been associated with the release of contaminants during reflow. These contaminants may potentially induce drift or wavelength shift in the optical components. Accordingly, there exists a need for an improved optical component package.